For reducing a volume of sewage sludges, a technology in which the sludges are dried and then combusted at a high temperature in a slagging combustion furnace has been already put to practical use. Further, a technology in which combustible wastes are combusted without emission of toxic matter by a combination of a gasification furnace and a slagging combustion furnace is about to be put to practical use. The purpose of this gasification and slagging combustion system is to prolong landfill sites by converting ashes into slag, utilize slag which has been converted from ashes to pavement materials or the like, decompose harmful substances such as dioxins completely, and establish a combustion technology which is suitable for environmental conservation, has a simple structure and low plant cost, yet has the above-mentioned functions.
FIG. 6 shows an example of a conventional gasification and slagging combustion system. As shown in FIG. 6, the gasification and slagging combustion system comprises a constant feeder 1, a fluidized-bed gasification furnace 2 and a swirling-type slagging combustion furnace 3. The fluidized-bed gasification furnace 2 has an air chamber 5 at a lower portion thereof and the air chamber 5 has an air diffusion plate 4 at an upper portion thereof. A fluidized-bed 6 of silica sand is formed over the air diffusion plate 4. A freeboard 7 is provided above the fluidized-bed 6 for preventing silica sand from being carried over and suppressing pressure fluctuations. On the other hand, the swirling-type slagging combustion furnace 3 has a primary combustion chamber 8, a secondary combustion chamber 9 and a slag separation chamber 10 therein.
Silica sand is located over the air diffusion plate 4 in the fluidized-bed gasification furnace 2, and air "b" supplied into the air chamber 5 is ejected upwardly from the air diffusion plate 4 to thus form the fluidized-bed 6 of silica sand over the air diffusion plate 4. The silica sand comprises river sand having a diameter of about 0.5 mm.
Combustible wastes "a" supplied into the fluidized-bed gasification furnace 2 by the screw-type constant feeder 1 fall into the fluidized-bed 6 which is kept at a temperature ranging from 450 to 850.degree. C., and are contacted with the heated silica sand and quickly pyrolyzed, thus generating gas, tar and fixed carbon. Then, these pyrolyzed substances are gasified by being contacted with oxygen in air "b". In the meanwhile, the fixed carbon is gradually pulverized by oxidization and a stirring action of the fluidized-bed.
Air "b" is blown into the freeboard 7 of the fluidized-bed gasification furnace 2, if necessary, and hydrocarbon, tar and fixed carbon are partially combusted at a temperature ranging from 650 to 850.degree. C. Large-sized incombustibles "d" are discharged together with silica sand from the bottom of the fluidized-bed gasification furnace 2. The discharged incombustibles "d" contain metals such as iron, copper or aluminum. As the inside of the furnace is in a reducing atmosphere, metals can be recovered in a non-oxidized and clean condition. The discharged incombustibles and silica sand are separated from each other by a separating device (not shown), and the large-sized incombustibles are discharged to the outside of the separating device and the small-sized silica sand is returned to the fluidized-bed gasification furnace 2.
The generated gas "c" discharged together with fixed carbon from the fluidized-bed gasification furnace 2 is supplied to the swirling-type slagging combustion furnace 3, and they are mixed with preheated air "b" in a swirling flow and rapidly combusted at a high temperature ranging from 1200 to 1600.degree. C. in the vertical primary combustion chamber 8, and the secondary combustion chamber 9 inclined slightly with respect to the horizontal. The combustion reaction is completed in the secondary combustion chamber 9. Because of the high temperature combustion, ash content in the fixed carbon is converted into slag mist which is mostly trapped by molten slag phase on an inner wall of the combustion chamber due to the centrifugal forces of the swirling flow. The molten slag "f" flows down on the inner wall and is discharged from the bottom of the slag separation chamber 10. Thereafter, the molten slag "f" is cooled indirectly or directly, and is then discharged as granulated slag to the outside of the furnace.
On the other hand, the exhaust gas "e" discharged from the top of the slag separation chamber 10 passes through a series of heat recovery equipment or dust removing equipment (not shown), and is then discharged to the atmosphere. In this manner, 90% of ash content is discharged as the molten slag "f" and the remaining 10% of ash content is mostly collected as fly ash by a bag filter.
In the conventional system shown in FIG. 6, after the combustion reaction is completed in the secondary combustion chamber, the molten slag is discharged from the furnace, and hence the primary combustion chamber is in a reducing atmosphere and the secondary combustion chamber is in an oxidizing atmosphere. Since slag produced in the secondary combustion chamber is exposed to the oxidizing atmosphere, vaporization of heavy metals having a low boiling point from the slag is not sufficiently performed.
To be more specific, municipal wastes and plastic wastes which are typical combustible wastes contain a trace of heavy metals having a low boiling point, such as Hg, Cd, Pb, Zn, or As, and the inclusion of such heavy metals having a low boiling point into the obtained slag is inevitable in the conventional gasification and slagging combustion system shown in FIG. 6. However, such heavy metals having a low boiling point entrapped in the slag are eluted out in an acid solution, and hence it is impossible to enclose the heavy metals having a low boiling point completely in the slag.
Further, in the complete combustion process in the slagging acombustion furnace, if wastes do not have a lower heating value of 2,000 kcal/kg or more, then auxiliary fuel is required. Therefore, there has been a need for lowering the heating value of wastes capable of being combusted independently. That is, there has been a need for such technology in which the lower limit of the heating value capable of operating the furnace without an auxiliary fuel can be lowered.
It is therefore an object of the present invention to provide a method for treating combustibles by slagging combustion which can obtain harmless molten slag whose content of heavy metals having a low boiling point is reduced to a level as low as possible, and can treat wastes without any auxiliary fuel even if the wastes have a low heating value.
According to a first aspect of the present invention, there is provided a method for treating combustibles by slagging combustion, characterized in that: combustibles and oxygen-containing gas are supplied to a slagging combustion furnace and the combustibles are partially oxidized in a reducing atmosphere to obtain combustible gas and convert ash content into molten slag which is discharged from the slagging combustion furnace; and the combustible gas is completely combusted by supplying oxygen-containing gas.
According to the first aspect of the present invention, since the process from formation of slag by melting ash content in the combustibles to discharge of the slag is carried out in a reducing atmosphere, vaporization of heavy metals having a low boiling point from molten slag into gas is accelerated, the amount of the heavy metals having a low boiling point remaining in the molten slag is reduced to the extremely low level, and harmless slag from which the heavy metals are not eluted out in a landfill site can be obtained. Thereafter, combustible gas obtained by partial oxidization is completely combusted by supplying an excessive amount of air or an excessive amount of oxygen-containing gas. In this manner, the wastes having a low heating value which could not be combusted without any auxiliary fuel in the conventional method can be melted without any supplemental fuel.
It is necessary that an amount of oxygen in the oxygen-containing gas supplied for partial oxidization of the combustibles is in the range of 40 to 100%, preferably 80 to 99% of a theoretical oxygen demand, and an amount of oxygen in the oxygen-containing gas supplied for complete combustion of the combustible gas is in the range of 30 to 90%, preferably 30 to 50% of a theoretical oxygen demand.
In this case, the combustibles comprises gaseous material and/or solid material obtained by partial oxidization of wastes in a gasification furnace by supplying oxygen-containing gas. Thus, even if the wastes are difficult to be pulverized like municipal wastes or plastic wastes, it is possible to treat the wastes by slagging combustion only by preparation of rough shredding of the wastes or the like. The partial oxidization of the wastes is performed in a bed having a temperature raging from 450 to 850.degree. C., preferably 450 to 650.degree. C., more preferably 500 to 600.degree. C. by using a fluidized-bed gasification furnace.
In this case also, a total amount of oxygen in the oxygen-containing gas supplied for partial oxidization of the combustible wastes and partial oxidization of the gaseous material and/or solid material is in the range of 40 to 100%, preferably 80 to 99% of a theoretical oxygen demand, and an amount of oxygen in the oxygen-containing gas supplied for complete combustion of the combustible gas is in the range of 30 to 90%, preferably 30 to 50% of a theoretical oxygen demand.
The slagging combustion furnace comprises a swirling-type slagging combustion furnace. The combustibles supplied to the swirling-type slagging combustion furnace are partially oxidized at a temperature ranging from 1200 to 1600.degree. C., and then the remaining combustible gas is completely combusted at a temperature of 900.degree. C. or higher.
According to a second aspect of the present invention, there is provided a method for treating combustibles by slagging combustion, characterized in that: combustible wastes and oxygen-containing gas are supplied to a gasification furnace and the wastes are partially oxidized to obtain gaseous material and/or solid material; the gaseous material and/or solid material and oxygen-containing gas are supplied to a slagging combustion furnace and the gaseous material and/or solid material are partially oxidized in a reducing atmosphere to obtain combustible gas and convert ash content into molten slag which is discharged from the slagging combustion furnace, and the combustible gas is completely combusted by supplying oxygen-containing gas.
According to the second aspect of the present invention, the wastes are gasified in the gasification furnace to obtain gaseous material and/or solid material, and the process from formation of slag by melting ash content in the gaseous material and/or solid material to discharge of the slag is carried out in a reducing atmosphere. Therefore, vaporization of the heavy metals having a low boiling point from molten slag into gas is accelerated, the amount of the heavy metals having a low boiling point remaining in the molten slag is reduced to the extremely low level, and harmless slag from which the heavy metals are not eluted out in a landfill site can be obtained. Thereafter, combustible gas obtained by partial oxidization is completely combusted by using an excess amount of air or an excess amount of oxygen-containing gas. In this manner, the wastes having a low heating value which could not be combusted without any auxiliary fuel in the conventional method can be burned without any supplement fuel.
In the second aspect also, it is necessary that an amount of oxygen in the oxygen-containing gas supplied for partial oxidization of the combustibles and partial oxidization of the gaseous material and/or solid material is in the range of 40 to 100% of a theoretical oxygen demand, and an amount of oxygen in the oxygen-containing gas supplied for complete combustion of the combustible gas is in the range of 30 to 90% of theoretical oxygen demand.
In the first and second aspects of the present invention, the sum of the oxygen amount in the oxygen-containing gas supplied for partial oxidization and the oxygen amount in the oxygen-containing gas supplied for complete combustion is in the range of 110 to 140%, more preferably 120 to 130% of atheoretical oxygen demand. As a gasification furnace for use in the present invention, a rotary furnace, a fluidized-bed furnace, or a fixed-bed furnace may be used. The fluidized-bed gasification furnace is preferable for treating the wastes because the size range of combustibles which can be used is wide. As a slagging combustion furnace, an entrained-bed furnace may be used, and further swirling-type furnace is preferable for high load combustion.